1. Technical Field
The invention relates to an optical measurement unit for a measurement device for the photoelectric scanning of a measurement object as well as a handheld photoelectric measurement device equipped with such an optical measurement unit. In particular, the invention relates to an optical measurement unit for a colour measurement device, a colour densitometer, a spectrophotometer or a multifunctional device for the quantitative capture of colours as well as to the device itself.
2. Background Art
The term handheld measurement device is understood in the following to be a generally autonomous, portable measurement device, which is normally provided with operating and display elements and is positioned manually at or on the object to be scanned. However, expressly included are also such devices which can be connected through a communication channel with an external device, for example a computer and can exchange measurement and/or control data therewith. In the extreme case, a handheld measurement device in accordance with the invention can include in a housing only an optical measurement unit and a downstream photoelectric converter arrangement.
Known handheld measurement devices of this type are, for example, “SpectroEye” of the company Gretag-Macbeth AG, “Vipdens 2000” of the company Viptronic, “530 SpectroDensitometer” of the company X-Rite and “SpectroDens” of the company Techkon.
Handheld measurement devices of this generic type are used for the spectral and/or densitometric colour measurement in a plurality of applications such as the packaging, newsprint, advertising, security, paint and lacquer industry. The measurable colour palette extends from the classical CMYK print colours through metallized up to highly pigmented ANIVA colours. They can be applied, for example, to substrates such as painted or unpainted paper and transparent or metallized foil. The broad range of applications and the sometimes rough environmental conditions place high demands onto a device used therefor.
A compact, ergonomic and robust construction is a basic requirement of a handheld measurement device for industrial use. Dust and dirt tightness are indispensable for the printing process control in the printing room where paint vapors and printing powder particles are suspended in the air. The measurement result must be as independent from the positioning of the measurement device as possible even for measurement objects with a finely structured and oriented surface structure. The demand for an ever smaller maculation by the printing of colour control fields is directly followed by a reduction in their dimensions. The measurement device must therefore be adaptable by way of an exchangeable measurement aperture to the colour control field size to be measured and must be exactly positionable thereon. Depending on the situation, a measurement without additional filters or with polarization or UV or D65 filters is required.
The mentioned handheld measurement devices currently on the market fulfill these demands to different degrees. A large problem common to all known devices consists however in the sometimes insufficiently precise, sometimes cumbersome positioning of the handheld measurement device on the measurement object, especially when the measurement locations to be scanned are very small.
Because of the often present surface texture of the measurement object, the impinging illumination light is not evenly diffusely remitted so that the measurement result is dependent on the positioning of the measurement device on the measurement object. This is especially the case with oriented illumination arrangements of simple construction. Positioning hereby refers to the rotational position of the measurement device relative to the normal on the measurement object at the measurement location. This problem is often addressed by an annular, which is all around continuous or made of discrete light sources. Because of its construction, it is the nature of an annular (45°/0°) illumination arrangement that the measurement location of the measurement object is covered along its whole circumference during the measurement. If such a measurement arrangement is used in a measurement device, more or less complex mechanisms are required to guarantee a clear line of sight to the measurement object during the positioning.
Several known devices (for example “Vipends 2000” and “530 SpectroDensitomer”) are equipped with a pivot mechanism. In the at rest position, in which the positioning of the device on the measurement object is carried out, the device housing is pivoted up relative to a base plate which includes a sighting opening, so that the sighting opening is freely visible. For the measurement, the device housing is then pivoted down, whereby the measurement head of the device is lowered onto the sighting opening. This can result in a shifting of the device so that the positioning is no longer correct. Furthermore, because of the pivot mechanism, such devices are mechanically relatively complex.
Other devices (for example, “SpectroEye”) have a motor driven measurement head which is located within the device housing during the at rest position and for the measurement is moved outside thereof and over a protruding sighting opening, and then returned again into the housing after the measurement. This solution is mechanically complex.
Further known devices (for example “SpectroDens”) have only a single sighting opening which covers the whole measurement location to be scanned so that the positioning of the measurement device especially with small measurement spots is rendered difficult. This and other devices mostly have a simple oriented illumination arrangement and are mechanically relatively simple. However, because of the absence of a continuous or discrete annular illumination, they have the mentioned measurement technological limitations.
It is now an object of the present invention to improve an optical measurement unit and a handheld measurement device of the generic type in such a way that with the smallest possible constructive cost a positioning independence is largely achieved and a precise positioning on the measurement object is guaranteed without cutbacks in the handling ergonomics.